Several types of pumps are especially useful for pumping liquids and other fluids with minimal back-flow and that are amenable to miniaturization. An example is a gear pump. Another example is a piston pump. A third example is a variation of a gear pump in which the rotary pumping members have lobes that interdigitate with each other. Gear pumps and related pumps have experienced substantial acceptance in the art due to their comparatively small size, quiet operation, reliability, and cleanliness of operation with respect to the fluid being pumped. Gear pumps and related pumps also are advantageous for pumping fluids while keeping the fluids isolated from the external environment. This latter benefit has been further enhanced with the advent of magnetically coupled pump-drive mechanisms that have eliminated leak-prone hydraulic seals that otherwise would be required around pump-drive shafts.
Gear pumps have been adapted for use in many applications, including applications requiring extremely accurate delivery of a fluid to a point of use. Consequently, these pumps are widely used in medical devices and scientific instrumentation. Developments in many other areas of technology have generated new venues for accurate pumps and related fluid-delivery systems. Such applications include, for example, delivery of liquids in any of various automotive applications.
Automotive applications are demanding from technical, reliability, and environmental viewpoints. Technical demands include spatial constraints, ease of assembly and repair, and efficacy. Reliability demands include requirements for high durability, vibration-resistance, leak-resistance, maintenance of hydraulic prime, and long service life. Environmental demands include internal and external corrosion resistance, and ability to operate over a wide temperature range.
A typical automotive temperature range includes temperatures substantially below the freezing temperature of water and other dilute aqueous liquids. These temperatures can be experienced, for example, whenever an automobile is left out in freezing winter climate. In contrast to many other substances, water and most aqueous solutions tend to expand as they undergo the phase change from liquid to ice. As is well known in household plumbing systems exposed to sub-freezing temperatures, the static pressures produced by freeze-expansion are sufficiently high to fracture pipe. Thus, these pressures can cause substantial damage to a pump that is coupled in a primed condition to a hydraulic circuit exposed to a sub-freezing temperature.
In view of the above, the simplest solution that might be proposed is simply to add anti-freeze to the liquid or to constitute the liquid with sufficient solute to depress its freezing point. Unfortunately, changing the liquid in these ways changes the composition and possibly other important properties of the liquid, which may render the liquid ineffective for its intended purpose. Hence, there is a need for pumps that can effectively withstand the internal pressure generated by a freezing condition without exhibiting damage that otherwise would be caused by freeze-expansion.
There is also a need for pumps that exhibit reduced pressure pulsatility of the output stream of liquid being pumped. Although many types of gear pumps, for example, deliver substantially continuous output streams, the output streams nevertheless tend to exhibit at least some pressure pulsatility that is synchronous with the rate at which increments of liquid between successive gear teeth are delivered downstream by the pump-head. Output-pressure pulsatility, a dynamic rather than static phenomenon, is exhibited by many types of pumps, including conventional gear pumps, piston pumps, and the like. Certain types of pumps, such as piston pumps, tend to exhibit a higher-amplitude output-pressure pulsatility than other types, such as gear pumps. Nevertheless, certain highly precise applications would be better served using pumps, otherwise highly effective for their assigned uses, that produce substantially less output pulsation than their conventional counterparts.